For nearly two decades now, Broadcast have been such a stellar presence and subtle revelation that the prospect of their no longer being around has yet to sink in. More than most, the group worked in their own time and space, a step aside from (and a league above) their co-travellers. So it feels, from this distance, oddly natural to be awaiting their final signals two years after Trish Keenan passed away. The good news is, there's another Broadcast album proper in the pipeline. The other good news is that we have this addition to their occult catalogue of offcuts, b-sides and experiments to tide us over.

There's a big essay waiting to be written about this group. Those analogue arrangements led reviewers up the retro reference path over the years, leaving the music's enigmatic operations under-explored. Those melodies, bold enough for Blondie, timeless as lullabies or folk songs. Those lyrics, intimate and economical, inviting you in and spiriting you away; playing with time, location, their own form. That astonishing soundworld: sumptuous, fierce, alien-beautiful, mutant-funky, always unexpected. Summoned through old equipment and arcane ideas, the whole show could make latest-tech electronica and pop peers alike sound played out – a quirk that seems less counterintuitive now that the digital dimension is such a prosaic fact of life.

As you might expect, that concentrated hit of surreal pop complexity is less forthcoming here than on previous records (the aim being to accompany the action rather than provide it all). Nonetheless Berberian Sound Studio is a nice confection with some transcendent moments that'll make you double-take. If you've seen the film you'll know that music doesn't dominate the soundtrack; its real stars are silence and the baroque screams and sound effects we see being performed. Broadcast's 'OST' was primarily written for The Equestrian Vortex, the unseen witches-and-goblins giallo around which the film's action is based. So what we're listening to here is non-diegetic sound designed as diegetic sound that the film's audience can only experience non-diegetically.

Given this and the staggering diversity in giallo music, Broadcast had free rein (ho ho) to take off in any number of directions. Overall, they've gone for a gentle, plaintive tone; themes that suggest tragic fates, distant diabolism and soft-focus sequences. Tracking the character names, you notice a slide between catchy hackwork and a deliciously strange late Broadcast sound. Collatina gets the prettiest ident, a descending flute line accompanied by one-note, period-piece 'la-la-la' backing vocals (the main motifs become proper earworms). But she also ascends to a Coil-like plane as a floating vocal ghost on 'Collatina, Mark of Damnation'. Its strange bob rises again in 'Teresa, Lark of Ascension', where it's weirded still further out by keening keys. Monica doesn't seem to have had any easier a time of it in The Equestrian Vortex than those two. She's a more ethereal presence even than Teresa, but the slight, seductive concoction 'Monica's Burial (Under The Junipers)', just a minute of echoed speech, bowed saw-like keyboards and stray sounds from a room, knocks you off balance.

There is some broadly retro styling – a heavy synth drone here, a churchy organ there – as the gig demands. But you'd have a hard time identifying nods to any particular horror soundtrack from the era (Morricone's alone contain a multitude of styles). Despite their sound palette, reference and pastiche have never been prominent in the Broadcast picture; neither were they a programmatic, disciplined band in the mould of Stereolab. So it comes as no surprise when their curiosity and ear for beauty gets the better of them; 'Our Darkest Sabbath', for instance, sounds like lights bleeding into each other – more Eno than Ennio. There aren't many groups whose lightest listening suite is the score for a horror film of sorts, but there you go.

Looking back, it's hard to imagine director Peter Strickland considering anyone else for the gig. From his film's inspiration – the meeting of avant-garde music and pulp cinema – it's a short leap to Broadcast's kitting out of pop songs with sonic tricks deployed by some of the composers involved. The supernatural has also been a regular visitor to Broadcast's world since the start ('Phantom', 'Valerie', their tribute to the surrealist-sensationalist vampire film and novel, 'Black Cat'), with witchcraft coming to the fore more recently on Witch Cults of the Radio Age. Too few people have had the pleasure, but the tour-only EP Mother Is The Milky Way took this tendency to its limit; it's a kind of lo-fi pastoral horror sound drama, all the more unnerving for being barely there.

Back to the film's pitch: a fusty Englishman bringing witches to life via lovingly shot analogue sound equipment, in a reimagined 1976. If you're sensing a further brand tie-in, it might be time to put a hex on a miniature cult: 'hauntology'.

Speaking as someone drawn to the uncanny across culture of all vintages, it's been dispiriting to see this critic's egg belatedly pushed in Broadcast's direction. The word itself is a pug-ugly tautology, the 'ontology' part already explicit in the spectral sense of the word 'haunt', leaving that quasi-academic 'ology' hanging like a spare blogger at a wedding. Yes, we know it's a pun by Derrida. But it would take a tin ear to think it translates, let alone travels lightly from the teetering text of Spectres of Marx to the music it's been forced upon.

There's the ontology: a theme either heavy enough to crush poor Belbury Poly and their niche retro pootling, or so banal it could apply to anything. There's the politics: if we're being haunted by post-war social democracy, where do The Caretaker's 1930s ballrooms fit in? There's the awkward fact that memory, nostalgia and the supernatural have always been present in pop – so what's in and what's out?

Because the traits touched upon are both too broad and too disparate to be tied up in a word, 'hauntology''s champions end up pulping their subject matter to feed their theory. Like the similarly détourned 'psychogeography', it deadens the senses to the qualities (you'd hope) it was intended to reveal. Both are cases of theory slipping into ersatz theology – fated attempts to capture elusive, unspeakable experiences in yards of self-referential explication guaranteed to stamp out their pleasures. There's nothing wrong with men in their forties enjoying shared tastes and nostalgic triggers, but do they have to be so po-faced about it? Do they really need their peccadilloes underwritten by such a painfully overwritten non-ology?

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